Nowadays, the knowledge about the new technologies is indispensible for companies that depend on them to be developed as well as to keep ahead of the competitors. The main goal of this paper is to present an availability study that allows the integration of the Satellite Communications with the Terrestrial Mobile Network (UMTS), providing global roaming to the T-UMTS (Terrestrial Universal Mobile Telecommunications System) users. The accomplished simulation shows the availability of fragmenting the PDUs (Protocol Data Units) to improve the throughput in the satellite link.